Morphogenesis shapes the adult organism and is exquisitely regulated during development. Misregulation of morphogenetic processes such as cell shape changes and cell migration can lead to diseases such as cancer. Unraveling the molecular mechanisms controlling morphogenetic events during normal development will also reveal what transpires when these processes go awry in disease states. Late in Drosophila embryogenesis, during the coordinated dorsal movement of epidermis known as dorsal closure (DC), multiple signaling pathways (Wg/Wnt, JNK, TGF-beta, and Notch) and the small GTPases (Rac, Rho and Cdc42) are collectively required for the epidermal cells to reorganize the cytoskeleton, change shape, and form a leading edge. DC is amenable to live cell biological analyses and is ideal for the application of a novel methodology to visualize subcellular localization of protein interactions in living cells. Fusing two non-fluorescent fragments of YFP to two proteins of interest whose interaction will bring the YFP fragments together to reconstitute fluorescence provides a marker for the complex. Since Rac is central to the network of signaling involved in DC, this approach will be used to visualize Rac's interactions and will establish its relationship to the network of signaling that controls cell shape changes. Additionally, cell-based high-throughput screening using RNA interference technology now affords the ability to perform genome-wide screens to identify novel genes based on phenotypes and will be employed to identify novel components of Rac-mediated cell shape changes. Together these approaches will provide insight to the mechanisms regulating morphogenesis.